It is described in the first-mentioned publication that during a control action the power piston of the master cylinder is brought into a defined position which guarantees that a sufficient volume of pressure fluid remains in the master cylinder so that the vehicle still can be braked to a sufficient extent in the event of failure of the pumps. The defined position is referred to by "B" in the publication. In the course of a control action, the power piston, and hence the pedal, are brought into a defined position which is independent of the pedal effort being brought to bear by the driver. The relation between the pedal travel and the brake pressure and, respectively, master cylinder pressure during a non-controlled braking action which is known to the driver is, thus, lost.
U.S. Pat. No. 4,892,364 suggests accomplishing the pedal positioning by variation of the delivery volume of the pump, and particularly to change the drive power of the pump motor. It is known from the French specification to vary the effective output of a pump in an anti lock system in that, with the drive power of the pump motor remaining constant, a by-pass between the pressure side and the suction side of the pump is opened so that the pump delivers in a circuit, and effective fluid delivery into the brake circuit does not take place.